The present invention relates to a semiconductor absolute pressure transducer assembly having a covering member mounted on a silicon diaphragm assembly on which semiconductor pressure sensitive elements are constructed, therefore, a vacuum chamber being provided therebetween. More particularly, the present invention relates to a semiconductor absolute pressure transducer assembly which is fabricated using a method for obtaining ideally hermitic seal between the silicon diaphragm assembly and the covering member.
In an ordinal semiconductor absolute pressure transducer assembly, semiconductor pressure sensitive elements such as piezoresistive bridge circuit are constructed on a silicon diaphragm assembly, each element of which varies its resistance value depending on pressure-induced strains, and a glass substrate is mounted on the surface of the silicon diaphragm assembly, thereby a vacuum chamber being formed therebetween. In electric signal, therefore relative to pressure applied to the silicon diaphragm, is obtained as an output. In manufacturing such pressure transducer assembly, it is considerably important to obtain perfectly hermetic seal between the silicon diaphragm assembly and the glass covering member, and simultaneously to fabricate a silicon diaphragm in which the pressure-induced strains appear uniformly on the whole surface in response to the pressure applied thereto.
In U.S. Pat. Nos. 3,918,019 or 4,079,508, there is shown a semiconductor pressure transducer assembly having a glass substrate and a thin silicon diaphragm upon which is diffused a piezoresistive bridge circuit. Bridge circuit components are properly oriented on the surface of the silicon diaphragm and connected to bonding pads and conducting paths, both of which are formed on the silicon diaphragm. The glass substrate has a circular well formed therein having a diameter at least as large as the diameter of the diaphragm. Conducting leads are disposed on the glass substrate in a pattern matching that of the bonding pads on the silicon. The silicon is bonded onto the glass substrate with the silicon diaphragm overlaying the well in the glass and bonding pads overlaying the conducting leads deposited on the glass using Anodic Bonding method. The bond performed by that method provides a hermietic seal between the silicon diaphragm and the glass substrate.
Generally, layer of insulating material such as silicon dioxide (SiO.sub.2) is provided on the surface of the silicon diaphragm for the purpose of protecting the piezoresistive bridge circuit and the conducting paths formed thereon. In such construction, however, it is very difficult to bond the insulating material such as glass substrate on the surface of the silicon diaphragm assembly under perfectly hermetic condition using a method such as Anodic Bonding.
In U.S. Pat. No. 3,595,719, method for bonding an insulator member to a passivating layer covering a surface of a semiconductor is described. In this method, etching of passivating layer is performed to a thickness of at least about 1,000 A. with an etchant prior to bonding.
In the case that such method is adapted for manufacturing of the transducer assembly mentioned above, the characteristic of the piezoresistive bridge circuit is harmfully affected because of high voltage applied across the p-n junction thereof. Namely, the silicon diaphragm assembly is n-type and pressure sensitive resistors of the piezoresistive bridge circuit which are diffused thereon are p-type. When the high voltage is applied between the silicon diaphragm and the glass substrate, with the positive voltage to the silicon diaphragm and the negative one to the glass substrate, then leak current flows through the p-n junction, thereby the insulating characteristic of the p-n junction of the piezoresistive bridge circuit becoming inferior.
Further, the conducting paths are formed by p+diffusion which yields a low resistivity of the n-type silicon. However, this diffusion also causes grooves along the constructed conducting paths, though they are very shallow in the depth. And the grooves have influence upon the hermetic seal between the silicon diaphragm and the glass substrate.
Another prior art is as follows:
(1) U.S. Pat. No. 3,397,278, "ANODIC BONDING":
This relates to Anodic Bonding technique. PA1 This shows the construction of a conventional pressure transducer.
(2) U.S. Pat. No. 3,968,466, "PRESSURE TRANSDUCER":